1. Field of the Invention
The invention relates to an electric vehicle and, more particularly, to an electric vehicle equipped with an electric motor outputting regenerative torque to a drive shaft connected to an axle shaft and a hydraulic brake providing braking force for a vehicle, and to a control method for the electric vehicle.
2. Description of Related Art
An electric vehicle in which a vibration suppression control is executed if a sharp change in regenerative torque is detected while a vehicle-driving electric motor is outputting regenerative torque has been proposed (see, e.g., Japanese Patent Application Publication No. 09-215106 (JP 09-215106 A)). In this vehicle, if a sharp change in regenerative torque is detected, a vibration suppression correction torque is calculated. The vibration suppression correction torque is the torque that will cancel out the fluctuation of the electric motor rotation speed caused by the sharp change in regenerative torque. Then, the vibration suppression control is performed by setting a torque command obtained by adding the vibration suppression correction torque to a regenerative torque that is demanded of the electric motor. By performing the vibration suppression control as described above, vibration at the time of a sharp change in regenerative torque is restrained.
However, in the foregoing electric vehicle, since the vibration suppression control is performed at the time of occurrence of a sharp change in the regenerative torque, vibration of the vehicle is restrained after the vibration has occurred. That is, vibration of the vehicle may not be able to be quickly restrained or may become large. On the other hand, it is also conceivable to always perform the vibration suppression control regardless of the presence or absence of a sharp change in regenerative torque. However, this may increase energy consumption and leads to deterioration in energy efficiency.
Some electric vehicles perform a regeneration coordination switching control in which the regenerative torque from the electric motor is switched to the braking force from the hydraulic brake. The regeneration coordination switching control is executed by gradually decreasing the regenerative torque while gradually increasing the braking force provided by the hydraulic brake. The regenerative torque is regenerated by the electric motor, which normally is high in response and therefore can be controlled relatively accurately. On the other hand, the braking force provided by the hydraulic brake, which normally is low in response and may not be accurately controlled in comparison with the electric motor. Therefore, because of the difference in response speed, communication lag and the like, the switching control may not be carried out a switch between equal amounts and may be cause of vibration to occur on the vehicle.